1. Field of the Invention
The present invention relates to an aqueous lubricant composition and, more specifically, to a hydrous type lubricant composition to be used for hydraulic fluid, metal cutting fluid, metal working fluid and so on.
2. Description of the Prior Art
It has heretofore been widely practiced to use water-base lubircants for hydraulic fluid, metal cutting fluid, metal working fluid and so on, since they do not cause fire accidents due to leakage thereof etc.
Water-base lubricants represented by a non-inflammable hydraulic fluid based on water/glycol, soluble type cutting fluid and so on contain usually a water-soluble high polymer compound such as polyether etc. as a lubricant and as a thickener in an amount of 15-40% by weight. In recent years, economizations in resources and energy became a social interest and it has thus become desirable to find an aqueous lubricant having higher water content. This request has been marked especially for non-inflammable hydraulic fluid based on water/glycol, namely a high water base fluid (HWBF) having a water content over 90% by weight, far beyond that (about 40% by weight) of conventional water base hydraulic fluid.
The increase in the water content up to over 90% by weight will inevitably cause a decrease in the concentration of the thickener in the hydraulic fluid which can no longer be cured by the conventional thickeners. Thus, for example, a water-soluble polyether having a molecular weight below 50,000 employed heretofore principally as the thickeners for non-inflammable hydraulic fluid based on water/glycol is stable against the mechanical shearing stress, but it exhibits low thickening effect and offers no greater increase of the viscosity on addition in a small amount, resulting in a larger leakage from seams of the machine due to the inferior lubricating power, so that it is not suitable as a thickener for the hydraulic fluid. It has also been proved, that a polyethylene oxide having a molecular weight within the range from several hundred thousand to several millions exhibiting a marked thickening effect even in a small addition amount is liable to undergo breakdown of the molecular weight due to the trend of being subject to molecular scission under a mechanical shearing stress, so that it may suffer a considerable lowering of the viscosity during operation and is not suitable as a thickener for water base hydraulic fluid.